The principle of decontamination and of sterilization through exposure to ultraviolet light pulses or flashes has long been known.
This technique makes use of the bactericidal efficiency of ultraviolet rays contained in intense white light flashes.
The decontaminating efficiency of pulsed light has been demonstrated on a wide range of micro-organisms: bacteria, molds, viruses, etc. The portion of UV radiation (between 200 and 300 nm) included in the flash results in, by the combination of a photothermal effect and a photochemical reaction, the destruction of micro-organisms present on the product subjected to the treatment.
The photochemical effect results from the absorption of UV rays by micro-organisms DNA. This absorption has maxima of about 200 nm and 247 nm, respectively. It causes disruptions and the formation of abnormal bonds within strands of DNA molecules, which prevent the replication thereof.
The photothermal effect is due to the fact that radiations absorbed by micro-organisms cause an abrupt increase in temperature. The absorption of received energy causes the disruption of cell membranes and the destruction of micro-organisms. The duration of this peak temperature, being a function of the duration of the light pulse, can be extremely short, typically in the order of a few hundred microseconds. The temperature rise (up to 150° C.) occurs at the microscopic level, whereas the temperature of the treated product does not substantially increase at the macroscopic level.
According to numerous works such as those of Wuytack et al. (Wuytack, E. Y., Thi Phuong, L. D., Aertsen, A., Reyns, K. M. F., Marquenie, D., De Ketelaere, B., et al., “Comparison of sublethal injury induced in Salmonella enterica serovar Typhimurium by heat and by different non thermal treatments”, Journal of Food Protection, 2003, vol. 66, pp. 1071-1073), the association of two effects, that is photochemical and photothermal effect, would explain the efficiency of the pulsed light treatment.
Usually, light pulses are generated by means of xenon flash lamps. Flash lamps are arc lamps operating in a pulsed mode. The electrical energy is accumulated in an electrical capacitor. A high voltage (several tens kV) signal triggers arcing in the gas contained in the lamp. The released of electrical energy in turn produces a light emission through gas ionization. Xenon is the most efficient inert gas for converting electrical energy into light energy, in particular in the UV range. For gas pressures used (equal to or higher than 1 bar), the spectrum of flashes is continuous. It consists of wavelengths from UV to near infrared (200 to 1100 nm). The duration of these flashes is typically in the order of one hundred microseconds or more.
The decontamination level obtained is dependent on the number and the power of applied flashes, the nature of the treated surface, and ranges from the mere logarithmic reduction (destruction of a fraction of micro-organisms) to the full sterilization (almost complete destruction of micro-organisms).
U.S. Pat. No. 4,910,942 to Dunn et al. is known, wherein the authors give details about conditions for implementing the decontamination technique by pulsed light flashes for applications in the food processing and pharmaceutical industry: food packages, drug packages, plastic films, liquids, food (fish, cheese, cakes). The authors also give details about the decontaminating effect obtained in different cases as a function of the number of flashes and the energy (in joules) per flash.
The decontamination technique by pulsed light flashes is an athermal process at the macroscopic level, in that the implemented physical mechanisms (photochemical and photothermal effects) are macroscopically essentially athermal, and therefore do not cause on their own a substantial rise in the average temperature of the decontaminating objects. This property is important for the treatment of heat sensitive objects, and in particular for preserving organoleptic properties of food.
However, when implementing a decontamination technique by pulsed light flashes, the warming generally turns out to be an important problem which causes annoying limitations. This warming comes from the light energy absorption by objects to be decontaminated, but also and above all by the machine parts exposed permanently to the radiation and which contact these treated objects. The thermal effect of the radiation is all the more high as the xenon flash lamps used for practical and economical reasons emit more than 85% of their energy spectral density in the visible and infrared part of the light spectrum.
The thermal effects are generally limited in prior art devices on the one hand by limiting the illuminating rates, and thus the object treatment rates, and on the other hand by using lamps with excitation energy levels which enable the spectral part of UV radiation to be optimized, however at the expense of a high reduction in their lifetime.
The purpose of the present invention is to provide a cooled pulsed light treatment device, which allows high treatment rates compatible with needs for a use in a production line, of heat sensitive objects, while optimizing the operating costs.